On a personal note, about 12 years ago I was diagnosed with Celiac Disease, an autoimmune disease that destroys the intestinal lining when wheat, barely, rye is consumed. 

The disease presents itself with many different symptoms or sometimes even no symptoms.  This is why it is so rarely diagnosed.  There is an easy blood test that can test for the disease.  The cure is a life long gluten free diet (wheat, barely and rye).  Blood tests have shown that approximately 1 in 133 people have the disease; that hardly classifies it as rare.  Unfortunately the average diagnosis is 9 years from the presentation of symptoms.

My wife is an "evangelist" of diagnosis; please don't be offended if she asks you about certain "bodily functions."

Here is a short list of symptoms somewhat in the order of predominance:

Anemia/ lack of stamina
Osteopenia, Osteoporosis
No weight gain even with massive calorie intake (yet 40% of newly diagnosed Celiac are overweight)
Massive, oily bowel movements
GERD (gastroesophageal reflux disease)
diagnosed with IBS (irritable bowel syndrome, IBS really stands for the doctor saying "I've Been Stumped")
cold sores/canker sores
peripheral neuropathies (numbness/tingling in hands and/or feet)
migraine headaches

So if you are really thin and at an age where putting on extra pounds is normally easy (over 40), my wife is likely going to take you aside and ask if you are anemic, have osteoporosis, bowel problems, etc.  Please don't be offended; she is making a diagnosis and is concerned that you get tested for Celiac Disease.

If the answer is "yes" to some of the above symptoms, we highly recommend getting tested for Celiac Disease.  Your doctor may insist that it is a rare disease.  1 in 133 is not rare.  The disease is so common in Scandinavia that you can order a Gluten Free hamburger in McDonalds !  If you are from a Northern European descent the 1% disease rate is on target.  If you are Asian or Black, you probably don't have this genetic defect.

While the Gluten Free diet is a hassle, the improved life is dramatic.  Many  have suffered for years before being properly diagnosed and respond within weeks of the Gluten Free diet.